1. Field of the Invention
This invention relates to a method and apparatus for detecting a radio frequency (RF) signal transmitted between an implantable medical device (IMD) and an external medical device in a telemetry session and for discriminating the transmitted RF signal from transient and steady state noise corrupting it.
2. Description of the Prior Art
In the field of programmable IMDs, it has become common to provide an interactive, transceiver system for both remotely programming operating functions, modes and parameters of the implanted device, and for telemetering out data related thereto on command by RF telemetry to an external medical device, commonly denoted a "programmer". Such IMDs include cardiac pacemakers, cardiac and other physiologic monitors, implantable drug dispensers, nerve, muscle, and brain stimulators of various types, cochlear implants, blood pumps, cardiomyostimulators, and tachyarrhythmia-control devices, e.g., implantable cardioverter/defibrillators (ICDs) for delivery of staged therapies to the ventricles and/or the atria, etc.
At the present time, both analog and digital information or data is typically transmitted by uplink RF telemetry from such IMDs to the external programmer upon receipt of a downlink telemetry interrogation command from the external programmer. The analog information has typically included battery voltage, physiologic signal amplitudes sensed in real time from sensors or sense electrodes, e.g., sampled cardiac electrocardiogram or EGM amplitude values, and, in the case of implanted pacemaker and ICD IPGs, pacing pulse and/or cardioversion shock amplitude, energy, and pulse width and lead impedance. Digital information includes digitized operating data, e.g., markers signifying device operations and data typically stored in RAM or ROM and transmitted in response to an interrogation command from such IMDs. Such stored data includes historic statistics related to device performance, episodic physiologic data stored in response to detection of an episode of interest or delivery of a therapy, e.g., cardiac electrogram segments, current programmed operating modes and parameter values, implant data, and patient and IMD identifier codes. Uplink telemetry is therefore employed to interrogate the IMD functions and memory and to confirm re-programming of operating modes and parameter values programmed in an downlink telemetry transmission.
Commonly assigned U.S. Pat. No. 5,683,432, incorporated by reference herein in its entirety, sets forth a history of the types of communication links that have been employed to communicate with an IMD, specifically including magnetic field coupling, reflected impedance coupling, and RF coupling. Static and dynamic magnetic field coupling techniques are only usable for limited programming of the IMD and have largely been abandoned, although use of static dynamic field coupling continues in the Medtronic Itrel II implantable neural stimulator. In a reflected impedance coupling system, information is transferred using the reflected impedance of an internal (implanted) L-R or L-C tuned circuit RF energized by an inductively coupled, external, L-R or L-C tuned circuit. Advantageously, such a system uses little or no current to transmit information. Disadvantageously, however, the maximum data rate of reflected impedance coupling systems is relatively slow, and the distance or rate at which information may be transferred is limited.
In RF coupled systems, which are perhaps the most commonly employed communication systems in modem implantable device systems, the RF carrier is modulated with information and is transferred from a transmitting antenna L-R or LC tuned circuit to a receiving antenna L-R or L-C circuit. Generally speaking, the modulated RF carrier induces a voltage in the receiving coil that tracks the modulated carrier signal which is then demodulated in order to recover the transmitted data. An example of a pacemaker programmer for use with programmable cardiac pacemakers having RF telemetry capabilities is disclosed in U.S. Pat. No. 4,550,370, incorporated by reference herein in its entirety.
Significant attenuation of the uplink and downlink RF telemetry signals occurs because the stainless steel or titanium canister commonly used to hermetically enclose an IMD and its antenna coil acts as a low-pass filter for the transmitted RF signals. Uplink telemetry transmission power cannot be increased to compensate for such attenuation because IMD battery power consumption must be minimized. The attenuation increases as frequency is increased, and so communications systems that are currently used have a maximum frequency of less than 200 kHz, which limits data transmission rate. Depending upon the type of modulation and demodulation used in an RF communication system, the data or bit rate cannot exceed a predetermined fraction of the carrier frequency; otherwise, the ability to reliably distinguish between modulation representing a digital (binary) "1" from a digital "0" is compromised. As a result of these constraints, the transmission range through the canister is limited to about 2-3 inches. A wide variety of proposals have been advanced in the prior art involving relocation of the telemetry antenna coil to or use of different antenna types at a location outside the canister of the IMD and use of higher frequencies in the megahertz range to increase operating range and data transmission rate but they have yet to be realized.
Since the time that such telemetry systems first became available, IMDs have proliferated in types and successive models or generations of each type that have been steadily improved in longevity and designed with increased programmable functions and capabilities. At first, in some instances, a single external programmer was designed to function with a single type or family of IMDs that could not be used to program or interrogate other IMD types or families or new generations thereof. A new programmer would have to be provided to the physicians as successive programmable IMD models and IMD functions became clinically available. In some instances, this problem was perceived and dealt with by providing the capability of upgrading the programmer so that it could communicate with the newly available
IMDs and at least confirm the identity of the IMD during a programming session for safety and record keeping reasons before proceeding to the programming and interrogation functions.
Microprocessor-based programmers were developed by Medtronic, Inc. and other manufacturers which operated under the control of dedicated, plug-in ROM modules or cartridges to enable the operation of the programming and interrogation telemetry with regard to specific model or series of models of IMDs. In such systems, the programmer is incapable of communicating with a given IMD model unless the appropriate plug-in module or cartridge is first installed. For example, for many years, particular Medtronic.RTM. MemoryMod.RTM. ROM cartridges were developed and supplied to enable the physician to upgrade the programmer to program and a-interrogate a specific set of new generation Medtronic.RTM. pacemaker implantable pulse generator models.
More sophisticated, computer based programmers have been developed that is also can be upgraded, including, for example, the Medtronic.RTM. Model 9710 and 9760 programmers and the more recent Medtronic.RTM. Model 9766 and 9790 programmers which employ the Medtronic.RTM. Model 9765 programming head. It is possible to load updated software for programming new generation IMDs onto a hard disk drive from floppy disks or compact discs or through a modem and many of the other alternative ways that programs are added to personal computers, for example.
Telemetry sessions between an IMD and the external programmer are typically initiated and conducted in the manner described in commonly assigned, U.S. Pat. No. 5,168,871, incorporated herein by reference herein in its entirety. Current telemetry systems are designed to provide two-way telemetry by RF signal transmission and linkage between an antenna coil contained in the IMD canister and an antenna coil or coils contained in the programming head of the external programmer. Typically, the programming head is placed against the patient's skin overlying the IMD, and a communications link is established as depicted and described in the above-incorporated '871 patent by closure of a reed switch within the IMD by the magnetic field of a permanent magnet incorporated into the programming head. Uplink telemetry of analog and digital data of the IMD and downlink telemetry of programming and interrogation commands to the IMD is conducted in a telemetry session according to a telemetry format that is related to the particular IMD.
The RF carrier signal is modulated with the data that is to be transmitted using a particular modulation or encoding scheme employed in RF communications. Such modulation or encoding schemes include FM and AM, phase shift keying (PSK), frequency shift keying (FSK), biphasic frequency shift keying (BPSK) amplitude shift keying (ASK), pulse position modulation (PPM), pulse interval modulation (PIM), among numerous others.
An extensive description of the historical development of uplink and downlink telemetry transmission formats and pulse encoding schemes are set forth in the following series of commonly assigned patents, all of which are incorporated by reference herein in their entireties. An example of a PIM telemetry scheme used for transmitting analog and digital data as binary "1" and "0" encoded intervals between successive RF pulses from an implanted pacemaker to a remote programmer is disclosed in commonly assigned U.S. Pat. No. 4,556,063. An example of a PWM telemetry system for transmitting binary "1" and "0" encoded RF pulse widths from an implanted cardiac pacemaker to an external programmer is described in U.S. Pat. No. 4,571,589. Commonly assigned U.S. Pat. No. 5,127,404 sets forth an improved method of frame based, PPM encoded data particularly for uplink telemetry to transmit more data per unit time and reduce implanted device current drain. The frame-based PPM telemetry format increases bandwidth well above simple PIM or PWM binary bit stream transmissions and thereby conserves energy of the IMD. Various PSK, FSK, BPSK, and ASK encoding schemes are described in the aboveincorporated '432 patent or in U.S. Pat. No. 4,698,111.
Commonly assigned U.S. Pat. No. 5,168,871 sets forth an improvement in the telemetry system of the '404 patent for detecting uplink telemetry RF pulse bursts that are corrupted in a noisy environment. U.S. Pat. No. 5,292,343 sets forth a further improvement in the telemetry system of the '404 patent employing a hand shake protocol for maintaining the communications link between the external programmer and the IMD despite instability in the programmer programming head. U.S. Pat. No. 5,324,315 sets forth an improvement in the uplink telemetry system of the '404 patent for providing feedback to the programmer to aid in optimally positioning the programmer programming head over the IMD. U.S. Pat. No. 5,117,825 sets forth an further improvement in the programmer programming head for regulating the output level of the magnetic, H field of the programming head telemetry antenna using a signal induced in a sense coil in a feedback loop to control gain of an amplifier driving the programming head telemetry antenna. U.S. Pat. No. 5,562,714 sets forth a further solution to the regulation of the output level of the magnetic, H field generated by the programming head telemetry antenna using the sense coil current to directly load the H field. U.S. Pat. No. 5,354,319 sets forth a number of further improvements in the frame based telemetry system of the '404 patent. U.S. Pat. No. 5,683,432 sets forth a communication system that dynamically adjusts operational parameters of the communication link during a telemetry session between an IMD and an external programmer.
Many of these improvements have been incorporated into current generation programmers which can be operated to program and interrogate long-lived IMDs that may be several generations old while also programming the most currently implanted generations of a wide variety of IMD types and families.
To ensure the safety of the patient, telemetry systems have been designed conservatively to avoid mis-programming or corruption of uplink telemetry data once acquisition has been obtained. The presence of electrical interference or noise strong enough to interrupt reception of telemetry from the external programmer can affect operation of uplink and downlink telemetry functions, including proper programming and interrogation. The programming and interrogation of IMDs typically takes place in hospital operating rooms, cauterization laboratories and physicians' offices which are often noisy electrical environments, and such noise has been found on occasion to interfere with the proper programming or interrogation of an IMD.
The programming heads used with the above-referenced Medtronic.RTM. Model 9710, 9760, and 9766 programmers, e.g., the Model 9765 programming head, employ antenna circuitry of the type described in commonly assigned U.S. Pat. No. 4,542,532, incorporated by reference herein in its entirety and in the above-incorporated '871 patent. The programming heads incorporate dual antenna coils tuned to 175 kHz center frequency to reject far-field noise which tends to link both coils with the same field strength. As explained in the '532 patent, the two antenna coils are wound in series opposition in the receive mode, and therefore, the noise field component from the remote noise source should be canceled, leaving primarily the signal component at the input of the receiver bandpass filter. However, noise is not always canceled in this manner, and it must be dealt with in the uplink telemetry receiver section of the programming head transceiver circuitry.
In one early approach, the signal output by the tuned in the receiver circuitry is bandpass filtered and processed using a manual gain control circuit. The use of automatic gain control (AGC) circuitry is disclosed in U.S. Pat. No. 4,562,840 and software implemented gain control circuitry is disclosed in U.S. Pat. No. 4,531,523, both incorporated by reference herein in their entireties. The gain of an amplification stage is attempted to be optimized to minimize mistaking noise and electrical interference for an uplink telemetry RF signal (characterized as a "false positive" response) and to avoid failing to pick up an actual uplink telemetry RF signal that is masked by such noise or interference (characterized as a "false negative" response).
These approaches do not completely eliminate false positive and false negative responses from occurring. Further improvements in the programming head receiver circuitry attempting to diminish these responses are disclosed in the above-incorporated '871 patent and were implemented into the Model 9765 programming head. In the '871 patent, the telemetry receiver section is coupled to a tuned circuit of the type described in the above-incorporated '532 patent, deliberately tuned outside the pulse frequency pass band. The receiver section bandpass filters and amplifies the signal output by the tuned circuit in response to RF signals and electrical noise or interference. Then, the filtered and amplified signal is applied to a detector block 124 incorporating signal phase shifting and mixing blocks. The phase shifting and mixing blocks invert positive polarity transient noise artifacts in the detector output signal applied to the carrier filter to reduce false positive response to positive polarity transient noise artifacts. The signal output by the detector block 124 is applied to a carrier filter block 130 that attenuates noise signals and artifacts and provides a demodulated uplink pulse signal that is then applied to an output comparator block 134 for comparison to a fixed reference voltage. The output comparator block 134 in the '871 patent compares the positive going demodulated uplink pulse signal with a fixed comparator threshold voltage that is set to 1.16 volts to provide the RCVTLM output signal to the programmer when the demodulated uplink pulse signal exceeds the threshold voltage. This relatively low threshold level is chosen to optimize overall performance by taking advantage of the characteristically clean base line of the receiver design. Under transient noise conditions, the detector block provides good suppression of false positive responses, but does not avoid false negative responses to actual uplink telemetry RF pulses. When a transient noise burst occurs coincident with an uplink telemetry RF burst, the phase relationship of the two signals will determine whether the amplitude of the positive going output signal (nominally 5.0 volts for the peak RF pulse) will be enhanced or degraded. The relatively low 1.16 volt comparator threshold provides added margin against false negative responses and is set at the level where both the false negatives and false positives begin to occur simultaneously for very large transient input noise levels, thereby optimizing overall receiver performance. However, the low amplitude comparator threshold is more likely to allow provide a false positive response if noise is not attenuated sufficiently by the detector block.
The comparator 134 also operates in a manner similar to a "slicer" commonly used in AM and FM signal demodulation that extracts analog signals from noise signals and converts the analog signals into a digital signal suitable for further processing by standard low voltage processing circuits. Programmer receivers that disclose use of an AM or FM "slicer" for analog to digital conversion of the analog signal are also shown in the above-incorporated '111 patent. In the '111 patent, the "slicer" simply comprises an op amp with feedback to provide hysteresis that compares the input signal to ground potential and provides a squared "binary" output signal.
These approaches do not solve all the false positive and false negative response problems due to noise in the signal output by the tuned circuit. At present, it is still necessary to hold the programming head as close to the IMD as possible and to maintain it very steady during the telemetry session to successfully telemetering data and commands between an IMD and an external programmer of the types described above. The uplink telemetry signal strength decreases exponentially with distance, and, consequently, noise levels become problematic as distance is increased. Moreover, the noise amplitude can vary considerably from moment to moment. In spite of these problems, it is a goal to be able to expand the coupling distance between the antenna coils in the IMD and the programming head, which necessarily reduces signal strength, while maintaining adequate noise discrimination. A primary reason for increasing the coupling distance is so that the programming head can be located out of the sterile field during an implantation of an IMD.
In addition, the uplink telemetry fidelity demands continue to steadily increase as the amount of uplink telemetered data is increased and as a greater variety of IMD types and models of any single type become available. The uplink telemetry signals output by these IMDs differ in type, frequency, pulse width and amplitude, modulation codes, etc., from manufacturer to manufacturer and sometimes between IMDs of differing models or types offered by the same manufacturer. As described above, considerable effort has been expended by manufacturers over the years to ensure at least that their current programmers operate with a wide range of both current and predecessor IMDs offered by that manufacturer. It is also desired to be able to make the programmer transceiver circuitry capable of accurately detecting a wide variety of uplink telemetry signals while avoiding or minimizing the false negative and false positive responses in the presence of electrical noise or interference.
Consequently, a need remains for further discriminating telemetry RF signals from noise and interference while expanding the capability of the telemetry system to operate with a wide variety of IMDs and increasing volumes of telemetered signals while relaxing the close and steady spacing requirements between the IMD antenna and the programmer antenna.